fix(todo-continuation-enforcer): check isContinuationStopped in injectContinuation to close race window

When /stop-continuation is invoked during the 2s countdown, the stop flag
was never checked inside injectContinuation, so the injection would still
fire after the countdown elapsed.

Propagate isContinuationStopped from handleSessionIdle through startCountdown
into injectContinuation, where it is now re-checked before any API call.

src/hooks/todo-continuation-enforcer/countdown.ts

@@ -38,6 +38,7 @@ export function startCountdown(args: {
   backgroundManager?: BackgroundManager
   skipAgents: string[]
   sessionStateStore: SessionStateStore
+  isContinuationStopped?: (sessionID: string) => boolean
 }): void {
   const {
     ctx,
@@ -47,6 +48,7 @@ export function startCountdown(args: {
     backgroundManager,
     skipAgents,
     sessionStateStore,
+    isContinuationStopped,
   } = args
 
   const state = sessionStateStore.getState(sessionID)
@@ -72,6 +74,7 @@ export function startCountdown(args: {
       skipAgents,
       resolvedInfo,
       sessionStateStore,
+      isContinuationStopped,
     })
   }, COUNTDOWN_SECONDS * 1000)